We've all had strange dreams: as a kid I was not in the least frightened by the Count Chocula character while awake, but I suffered through many a nightmare about him, his fangs dripping chocolate blood as he stalked me, Bela Lugosi-style, through the eerily empty halls of my school. God knows why. Other dreams make even less sense: I'm packing for a trip when there's a knock at the door. It's Fedex. For some reason, supreme court nominee Sandra Sotomayor has overnighted me a kitten.

So why does the brain produce this narrative junk? We still don't know why for certain, but the last few decades have produced a number of interesting evolutionary theories that move beyond old-hat Jungian archetypes or Freudian "wish fulfillment," and Scientific American's Jesse Bering recently laid out the Darwinian contenders.

Brain conditioning

If your brain went completely dark all night, the theory goes, it would begin to lose function just as rarely-used muscles will atrophy.

Several researchers, including the psychophysiologist Fred Snyder, argued that the adaptive purpose of dreaming may therefore be primarily to stimulate the brain or to keep it "in shape" during prolonged periods of inactivity. Later research offered support for this general idea. For example, specific categories of neurotransmitters were shown to be highly active during this period, while others seemingly "rested."

In other words, as psychologist Steven Pinker puts it, "Dreaming might be a kind of screen saver in which it doesn't really matter what the content is as long as certain parts of the brain are active."

External vigilance

Most dreams are notably lacking in olfactory and auditory content, and one theory holds that that's because if they were, the dreamer would be particularly susceptible to real-world threats like fire or noisy predators.

Being a "light sleeper" in relation to these other sensory domains had adaptive benefits, and since we're in the dark anyway and our eyes are closed, there's less of a risk in hallucinating in our secret visual worlds while our brains are being recharged.

Threat Simulation

This theory holds that dreams function as practice run-throughs for dangerous situations that may occur in the real world; they're drills. (Of course, this theory doesn't explain my kitten-in-the-mail dream; what was that preparing me for?)

"By giving rise to a full-scale hallucinatory world of subjective experience during sleep, the dream production mechanism provides an ideal and safe environment for such sustained practice by selecting threatening waking events and simulating them repeatedly in various combinations." What we should see in contemporary dreams, argues Revonsuo, are "threat scripts" depicting primitive themes of danger that would likely have been relevant in the ancestral environment, such as being chased, falling and so on.

Dreaming as problem solving

According to Harvard University psychologist Deirdre Barrett, "sleeping on it" really works in terms of real-world problem solving, and may actually be the evolutionary purpose of dreaming (even if those dreams don't always make sense to us.) In other words --

-- dreamscapes provided our ancestors (and therefore us) with a sort of creative canvas for solving real-world problems. In support of this, Barrett describes the work of Stanford University psychologist William Dement, who in the early 1970s instructed hundreds of undergraduate students to work on a set of challenging brainteasers before bedtime, so that they'd fall asleep with the problems still on their mind.